A known method of debris mitigation is the supply of a buffer gas between the EUV-radiation source and the collector. The debris particles, in the case of atoms or ions, slow down by the collisions with the gas atoms and are deflected from their original flight direction. With a sufficiently high density of the buffer gas the debris particles can be essentially stopped on their way to the collector. If the debris also contains condensable matter, for example metal atoms or metal droplets, an additional debris mitigation device is used between the EUV-radiation source and the collector. Such a debris mitigation device comprises a structure having passages for the straight pass of the radiation to the collector, wherein the debris material mainly condenses on the walls of this structure and therefore does not reach the collector.
Known debris mitigation devices are composed of several thin sheets or foils of a solid material arranged in a parallel, concentric or honeycomb structure forming manner, see for example WO 01/01736 A1. Such debris mitigation components are also called foil traps.
WO 03/034153 A1 discloses an embodiment of a debris mitigation device, in which the foil trap is separated by an intermediate space into two parts. The buffer gas is fed in this intermediate space. Due to this construction, the volume for plasma generation and the volume containing the collector can be maintained at a low pressure while the buffer gas in the intermediate space can be supplied with a higher pressure in order to effectively slow down the debris particles.
In order to effectively suppress the debris particles, a pressure of the buffer gas of several ten Pascal (cold pressure) over an interaction distance of several centimetres is required. The atomic weight of the buffer gas atoms should be similar to the atomic weight of the atoms and ions to be stopped in order to ensure the most effective momentum transfer.
EP 1 274 287 describes a debris mitigation device, in which the foil trap is rotated about the central optical axis. By this rotation, slowly moving, but relatively large and heavy droplets being nearly not deflected by collisions with the buffer gas atoms and therefore passing through the foil trap, can collide with the rotating foils of the foil trap and are mainly adsorbed by the surface of these foils.
In such a debris mitigation device a drive unit is necessary to rotate the foil trap. Such a drive unit comprises a driving motor and a driving axis to which the rotating foil trap is attached or fixed. The driving axis is usually supported by two bearings, for example ball bearings, in order to allow stable rotation of the driving axis and to ensure a precise alignment of the rotational axis. When using a debris mitigation device with a EUV radiation source, the radiation source together with the debris mitigation device and the collecting optics must be operated under vacuum conditions, wherein only the use of an additional buffer gas is allowed. In order to avoid contamination of the delicate optical surfaces of such a system, the purity requirements for the vacuum within the whole system are very high. This causes problems with the use of a drive unit. Already very small portions of for example hydrocarbons which are part of oil or grease vapors must be avoided. Therefore, the bearings of the driving motor cannot be oiled or lubricated and a special gas tight or vacuum-compatible driving motor must be used in order to minimize outgassing. Under such conditions the service life of the bearings is very short due to inevitable mechanical wear. For the bearings even when using selected materials and highest precision, only operation periods of some 100 h are achievable. Such short operation periods are not acceptable for applications in lithographic systems for semiconductor production.